This invention relates to vehicle bumpers and more particularly to a double beam configuration for motor vehicle bumpers.
Bumpers are used on motor vehicles, such as automobiles, to absorb light impacts and to prevent permanent damage from being caused to the automobile body during heavier impacts. A design objective of bumpers is to confine any permanent damage during impact to the bumpers which, presumably, can be replaced. The energy of impact must be absorbed or dissipated without permanently deforming the main structure of the automobile. In conventional designs the impact energy of light impacts is stored as elastic strain energy in the deformed bumper as well as in the automobile body. Conventional bumpers are designed to absorb the greater excess energy of heavier impacts by undergoing permanent plastic deformation, which can consist of small local dents or substantial deformations requiring the replacement of the bumper both for cosmetic reasons and in order to regain the lost protection against impacts. Such bumpers can absorb only very light impacts without undergoing some form of permanent deformation.
Existing automotive bumpers essentially are comprised of a beam (the bumper) on two supports, and are designed to absorb the energy of impact through elastic deformations. These bumpers all suffer from a major shortcoming, namely, for a given impact energy, the peak load experienced by the bumpers goes up drastically as the impact point moves closer to the support. Because of this phenomenon the bumper does not absorb any significant amount of energy for impacts squarely over the supports and, if shock absorbers are not used, such impact results in major irreversible damage to the bumper system.
In more recent designs, motor vehicle bumpers are connected to the vehicle body through shock absorbers which are capable of dissipating relatively large amounts of energy. As an example, the shock of impact on a bumper can be absorbed by pistons that force oil through small orifices, thereby dissipating large amounts of energy, and the assembly can be brought back to the initial state by means of springs. In this way it is possible to use relatively stiff bumpers that do not undergo local or global permanent deformations, but in which the bumper-piston assembly undergoes large displacements (stroke of piston forcing oil through small orifices), followed by complete recovery to the initial state. The cost of the bumper assembly is, however, substantial.
It is an object of the present invention to provide a motor vehicle bumper that is lightweight and relatively inexpensive and yet can sustain impacts of specified severity without undergoing permanent deformation.
It is a further object of the present invention to provide a plastic bumper that does not require shock absorber mounting to the motor vehicle body.